This invention relates to a master cylinder comprising a cylinder body, and a piston slidably mounted in the bore of the cylinder body while kept in slide contact with a cup fitted in the inner wall of the bore.
This type of master cylinder, which is typically used in a hydraulic brake system for a vehicle, has a pressure chamber into which brake fluid is adapted to be sucked from a reservoir. A primary cup or seal ring is in slide contact with the piston. In one conventional arrangement, brake fluid is sucked into the pressure chamber through a passage formed by elastically deforming the primary cup under the pressure of the brake fluid. When deforming the primary cup, the fluid encounters large resistance. This of course hinders smooth flow of brake fluid from the reservoir into the pressure chamber. Among brake systems including an electronic control unit for controlling brake hydraulic pressure, there are ones of the type in which brake fluid is sucked from the reservoir into the brake circuit through the pressure chamber. It is desired that the master cylinder used in this type of brake system be especially low in such suction resistance.
JP patent publications 10-513413 (Publication 1) and 2002-104162 (Publication 2) disclose master cylinders of the abovementioned type in which brake fluid is adapted to be sucked into the pressure chamber through a passage bypassing the primary cup so that brake fluid can be sucked without encountering any major resistance.
The master cylinder of Publication 1 includes a piston having an annular groove formed in its radially outer surface and lateral (radial) holes through which the annular groove communicates with the bore of the piston, which is a portion of the pressure chamber. The annular groove is formed in the piston at such a position that while the piston is not being operated, it is disposed behind the primary cup so as to communicate with a passage leading to the reservoir. Thus, while the piston is not being operated, the pressure chamber is in communication with the reservoir through the lateral holes and the annular groove, while bypassing the primary cup. Brake fluid can thus be smoothly sucked into the pressure chamber without encountering any major resistance.
The master cylinder of Publication 2 has a primary cup and a spacer both axially slidably fitted around the piston with the spacer disposed behind the primary cup. A guide member is further provided behind the spacer. The guide member is formed with a passage leading to the reservoir. The piston is formed with radial ports communicating with the pressure chamber. While the piston is not being operated, the radial ports communicate with the passage formed in the guide member, with their openings partially closed by the spacer and the primary cup. When a negative pressure is produced in the pressure chamber in this state, the primary cup and the spacer are pulled forward under the negative pressure produced in the pressure chamber. Thus, as soon as a negative pressure is produced in the pressure chamber, brake fluid can be smoothly sucked into the pressure chamber from the reservoir through the passage formed in the guide member and the radial holes because the primary cup and the spacer move away from the openings of the radial ports.
While the master cylinder disclosed in Publication 1 is simple in structure, the primary cup tends to be damaged by the edges of the annular groove when the piston slides. This may lead to premature breakdown of the seal.
The master cylinder produces pressure only after the piston has moved a distance corresponding to the width of the annular groove from its initial position. Since the annular groove has to have a width greater than the diameter of the lateral holes, the piston has to be moved a relatively long distance until the master cylinder produces pressure.
The piston disclosed in Publication 2 has no such annular groove, so that the master cylinder of Publication 2 is free of either of the above problems. But the master cylinder of Publication 2 needs the spacer and the guide member. The guide member has to be formed with the passage leading to the reservoir. In order to mount the spacer and the guide member, the cylinder body has to be formed of two separate members. All these factors extremely complicate the structure of the entire master cylinder.
There has been no conventional master cylinder that is free of either of the abovementioned two problems the master cylinder of Publication 1 has, and also does not need extra components such as the spacer and the guide member as required in the master cylinder of Publication 2.
An object of the invention is to provide a master cylinder that is simple in structure, can minimize the stroke of the piston required to seal the port, and minimize the possibility of the primary cup getting damaged.